The present invention relates to a magneto-optical recording medium and methods for recording information on and reproducing information from magneto-optical recording media. In more particular, the invention relates to a magneto-optical recording medium on which it is possible to carry out multi-valued recording by providing a plurality of magnetic layers in the medium or high density recording by recording information on each layer. The invention also relates to recording and reproducing methods and apparatuses for such a medium. The invention further relates to a magneto-optical recording medium suitable for high density recording which makes it possible to perform reproduction by magnifying a minute recording magnetic domain extremely smaller than a reproducing light spot. The invention also relates to a reproducing method for such a medium.
It is possible to rewrite the information recorded on magneto-optical recording media, which are large in storage capacity and highly reliable. Therefore, magneto-optical recording media begin to be practically used as computer memories etc. In view of an increase in amount of information and downsizing of devices and/or apparatuses, however, higher density recording and reproducing techniques are demanded. Various methods are suggested for producing high density magneto-optical recording media. One of the methods is called the multi-layer recording or multiple recording method for producing a high density magneto-optical recording medium by forming multiple recording layers in a magneto-optical recording medium, and recording information on each layer.
A system for recording multi-valued signals is known as described in, for example, Digests of the 13th Annual Conference on Magnetics in Japan (issued in 1989), page 63 and Japanese Journal of Applied Physics, Vol. 28 (1989) Supplement 28-3 pp. 343-347.
The multi-valued recording system involves stacking (or laminating) a plurality of magnetic layers having different coercive forces, and modulating in multiple levels or stages the strength of a magnetic field applied to the magnetic layers, thereby selectively inverting the magnetization of a specified magnetic layer. It is described that this system makes four-valued recording of signals possible by providing in a recording medium three magnetic layers having different coercive forces.
According to the multi-valued recording system for magneto-optical recording media, however, in order to detect for reproduction the multi-valued signals recorded on a magneto-optical recording medium, these signals have been distinguished by slicing at different levels the signals detected from the medium. Accordingly, it has been impossible to obtain large differences between the signal amplitudes corresponding to the multi-valued states, making it difficult to definitely distinguish two states close or near in signal amplitude. For this reason, the reproduced multi-valued signals have had a low S/N ratio. Therefore, there has been a demand for a reproduction technique for reproducing signals at a high S/N ratio from a magneto-optical recording medium with information recorded at high density.
There is conventionally known no magneto-optical recording medium having a plurality of magnetic layers on each of which information can be recorded and from each of which information can be reproduced independently. A recording medium having such performance may be very effective in recording various types of information correlatively on a single recording medium, or recording and reproducing pieces of channel information simultaneously in parallel.
When it is intended to reproduce information from a recording mark recorded at high density, a problem arises concerning the optical reproducing resolution which depends on the spot diameter of a reproducing light beam. For example, it is impossible to perform reproduction while distinguishing a minute mark having a domain length of 0.15 xcexcm by using a reproducing light beam having a spot diameter of 1 xcexcm. In order to eliminate such restriction on reproducing resolution resulting from the optical spot diameter of a reproducing light beam, an approach has been suggested concerning the magnetically induced super resolution technique (MSR) as described, for example, in Journal of Magnetic Society of Japan, Vol. 17, Supplement No. S1, page 201 (1993). This technique utilizes the occurrence of temperature distribution over a magnetic film in a reproducing light beam spot when a magneto-optical recording medium is irradiated with a reproducing light beam. A magnetic mask is generated in the spot so that the effective spot diameter, which contributes to signal reproduction, is reduced. The use of this technique makes it possible to improve the reproducing resolution without reducing the actual spot diameter of the reproducing light beam. However, in the case of this technique, since the effective spot diameter is decreased by the magnetic mask, the amount of light which contributes to the reproduction output is decreased, and the reproduction C/N is lowered to that extent. As a result, it is difficult to obtain sufficient C/N.
Japanese Patent Application Laid-Open No. 8-7350 discloses a magneto-optical recording medium comprising a reproducing layer and a recording layer on a substrate. Reproduction can be performed while magnifying the magnetic domains in the recording layer. When the magneto-optical recording medium is subjected to reproduction, an alternating magnetic field is used as a reproducing magnetic field to alternately apply a magnetic field in the direction for magnifying a magnetic domain and a magnetic field in the opposite direction, thereby magnifying and reducing the magnetic domains.
However, the multi-layer recording or multiple recording method involves recording information on or reproducing information from one recording layer with incident light intersecting (or crossing) another recording layer, which absorbs or disperses the light, thus reducing the amount of light contributing to the recording or the reproduction. As a result, it is difficult to obtain a sufficient S/N ratio.
The technique disclosed in Japanese Patent Application Laid-Open No. 8-7350 merely has the effect of improving the quality of signals for only a single recording layer. This technique does not have the densifying effect achieved with a plurality of recording layers laminated or stacked.
In view of the foregoing problems with the prior art, it is an object of the present invention to provide a magneto-optical multi-layer recording or multiple recording medium for recording information on each of its recording layers, the medium making it possible to record information thereon and reproduce information therefrom at a sufficient S/N even if minute magnetic domains are recorded on the medium. It is another object of the invention to provide a method and an apparatus for reproducing information from such a recording medium.
In accordance with a first aspect of the present invention, there is provided a magneto-optical recording medium comprising a plurality of recording layers on which data are recorded, and at least one reproducing layer to which the recorded data are transferred and which reproduces the transferred data.
FIG. 46 conceptually shows a magneto-optical recording medium according to the present invention. This recording medium includes three magneto-optical recording layers 11, 12 and 13. The data recorded on the recording layers 11, 12 and 13 are reproduced with reproducing beams 31, 32 and 33, respectively, which differ in wavelength. This makes it possible to densely record data and reproduce the densely recorded data in comparison with a magneto-optical recording medium including a single recording layer. Reproducing layers 21, 22 and 23 are provided for the recording layers 11, 12 and 13, respectively. The magnetization data in the recording layers are transferred to the respective reproducing layers. The magnetic domains transferred to the reproducing layers are magnified under an external magnetic field. Accordingly, it is possible to amplify and reproduce the data recorded on each recording layer. Therefore, by using the magneto-optical recording medium according to the present invention, it is possible to reproduce at a high C/N the data recorded densely on the two or more recording layers.
The foregoing magneto-optical recording medium may include reproducing layers for the respective recording layers. Alternatively, the recording medium may include a single reproducing layer to which a combination of states of magnetization in the recording layers is transferred. The recording medium may also include a blocking (intermediate) layer. When the magnetic domain transfers from one of the recording layers to the associated reproducing layer, the blocking layer blocks an influence of a leakage magnetic field from another of the recording layers. The blocking layer may be interposed between the recording layers or between at least one of the recording layers and the associated reproducing layer.
The magneto-optical recording medium according to the present invention may include a first recording layer, a second recording layer, a first reproducing layer and a second reproducing layer. The first and second recording layers are associated with the first and second reproducing layers, respectively. During reproduction, a magnetic domain of at least one of the first and second recording layers may be transferred to the associated reproducing layer. The transferred domain may be magnified under an external field. The magnified domains in the reproducing layers may be reproduced independently with reproducing beams different in wavelength. Alternatively, the recording medium may include a single reproducing layer to which a combination of states of magnetization in the recording layers can be transferred. In this case, a leakage magnetic field affecting the reproducing layer may vary in magnitude with the combination of states of magnetization in the recording layers. A magnetic domain transferred to the reproducing layer may be magnified to a size conforming to the magnitude of the leakage field.
The recording layers may have different compensation temperatures which are in the range between room temperature and the respective Curie temperatures. In this case, reproduction may involve radiating a first reproducing beam and a second reproducing beam which differ in power. The compensation temperature of one of the recording layers may be near to the temperature of the recording medium heated when the first reproducing beam is radiated. The compensation temperature of another of the recording layers may be near to the temperature of the recording medium heated when the second reproducing beam is radiated. The recording layers may include a first recording layer and a second recording layer. If reproduction involves radiating a high power reproducing beam PH and a low power reproducing beam PL, the radiation of the reproducing beam PH may transfer a magnetic domain of the second recording layer to the reproducing layer, and the radiation of the reproducing beam PL may transfer a magnetic domain of the first recording layer to the reproducing layer. The transferred magnetic domains can be magnified and reproduced by the MAMMOS described later.
In accordance with a second aspect of the present invention, a reproducing method is provided for reproducing the data recorded on a plurality of recording layers of a magneto-optical recording medium by radiating a plurality of different reproducing beams the recording medium to transfer the recorded data to a reproducing layer of the recording medium. This reproducing method makes it possible to reproduce through the reproducing layer the data recorded on the two or more recording layers. This makes it possible to realize a high density recording medium including a plurality of recording layers. The plurality of reproducing beams may differ in wavelength. In this case, the recording layers may be irradiated with the reproducing beams, which differ in wavelength, so that the data recorded on the recording layers can be reproduced independently at different wavelengths. The reason for this is that it is possible to apply to the present invention the multi-wavelength magneto-optical reproducing method described later. The reproducing beams may differ in power. In this case, the data recorded on at least one of the recording layers may be reproduced independently with at least one of the reproducing beams, which differ in power. The magnetic domains transferred from the recording layers to the reproducing layer may be magnified under an external field, which may be an external magnetic field, a light intensity-modulated reproducing beam, or a combination of them.
In accordance with a third aspect of the present invention, another reproducing method is provided for reproducing the data recorded on a plurality of recording layers of a magneto-optical recording medium. This reproducing method includes the steps of transferring a combination of states of magnetization in the plurality of recording layers to a single reproducing layer of the recording medium, and reproducing the transferred combination. A leakage magnetic field affecting the reproducing layer may vary in magnitude depending on the combination of states of magnetization in the recording layers. It is possible to magnify to a size conforming to the magnitude of the leakage field a magnetic domain transferred to the reproducing layer, and reproduce the magnified domain. That is to say, the size of the magnified magnetic domain may be correlated with a combination of states of magnetization in the recording layers so that recorded data can be reproduced in conformity with the size of the magnified domain. This makes it possible to record multi-valued data as a combination of states of magnetization in the recording layers, and reproduce the multi-valued data in conformity with the size of the magnified domain.
In accordance with a fourth aspect of the present invention, still another reproducing method is provided for reproducing data recorded on a plurality of recording layers of a magneto-optical recording medium. This reproducing method includes the steps of recording magnetic domains on the recording layers with different recording clocks, reproducing the recorded domains by transferring the recorded domains to reproducing layers of the recording medium and magnifying the transferred domains while applying external fields synchronously with the different recording clocks, and independently reading out from the magnified domains the data recorded on the recording layers.
In each of the reproducing methods according to the present invention, multi-valued data may be recorded in advance on the recording layers by the magneto-optical multi-valued recording method described later.
In accordance with the present invention, a reproducing apparatus is provided for reproducing, by the multi-wavelength magneto-optical reproducing method, the data recorded on a plurality of recording layers. In accordance with the invention, another reproducing apparatus is provided for reproducing, by the optical modulation domain magnifying and reproducing method, the data recorded on a plurality of recording layers.
The present invention employs the method (magneto-optical multi-valued recording method) disclosed in Japanese Patent Application Laid-Open No. 8-129784 filed by the applicant(s) of this application which makes it possible to record multi-valued data on a plurality of recording layers, the method (multi-wavelength magneto-optical reproducing method) disclosed in WO97/03439 for reproducing with reproducing beams different in wavelength the multi-valued data or the trains of data recorded on a plurality of recording layers, the method (optical modulation domain magnifying and reproducing method) disclosed in Japanese Patent Application No. 9-244845 for reproducing the data recorded on a recording layer, by transferring the recorded data with light intensity-modulated reproducing light to a reproducing layer and magnifying the transferred magnetic domains, and the method (magnetic magnifying and reproducing method or magnetically amplified magneto-optical system (MAMMOS)) disclosed in WO98/02878 for reproducing the data recorded on a recording layer, by transferring the recorded data with reproducing light to a reproducing layer and magnifying the transferred magnetic domains with an external magnetic field applied. These methods will be described briefly below, but the magneto-optical multi-valued recording method will be described in detail in Embodiment A8 and will therefore not be described now.
The multi-wavelength magneto-optical reproducing method is a recording and reproducing method for recording, on a magneto-optical recording medium including a plurality of magnetic layers, multi-valued data as combinations of states of magnetization in the recording layers, and reproducing the multi-valued data from the sum of the states of magnetization in the magnetic layers. This reproducing method is characterized by the steps of irradiating the magnetic layers with light beams having wavelengths xcex1 and xcex2 (xcex1, xe2x89xa0xcex2), respectively, converting the signals reproduced from the reflected beams of the different wavelengths into at least two-valued reproduced signals, and performing a logical operation for the converted signals of the different wavelengths to reproduce the recorded multi-valued data. This reproducing method may employ a magneto-optical recording medium wherein the strength ratios between the reproduced signals detected for the combinations of states of magnetization in the recording layers at the wavelength xcex1 differ from those at the wavelength xcex2.
Another magneto-optical recording medium may be employed wherein the order of strength of the reproduced signals detected for the combinations of states of magnetization in the recording layers at the wavelength xcex1 differs from that at the wavelength xcex2. This recording and reproducing method may employ a magneto-optical recording medium including two magnetic layers so that four combinations of states of magnetization in the layers make four-valued recording possible. The magnitude of each of the signals xcex81 to xcex84 reproduced from the four combinations of states of magnetization at the wavelength xcex1 differs from that at the wavelength xcex2. With the two magnetic layers irradiated respectively with light beams of the wavelengths xcex1 and xcex2, signals may be reproduced from the reflected beams. The reproduced signals may be sliced at one or more levels so that at least two-valued reproduced signals can be obtained. A logical operation may be performed for the at least two-valued reproduced signals from the two wavelengths so that the four-valued recorded data can be reproduced.
The principle of the multi-wavelength magneto-optical reproducing method will be explained briefly below. Explanation will be made as exemplified by reproduction of a four-valued recording signal recorded on a magneto-optical recording medium having two magnetic layers (recording layers) produced in Embodiment A1 described later on. FIG. 3(a) illustrates states of magnetization in first and second magnetic layers (recording layers) subjected to recording in accordance with Embodiment A4 described later on. Four combinations of states of magnetization (↑↑) (↑↓) (↓↑) (↓↓) exist on the magneto-optical recording medium based on the combination of the directions of magnetization in the first and second magnetic layers (The states of magnetization in the first and second magnetic layers are shown in that order). The four combinations of states of magnetization correspond to four-valued signals xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d, xe2x80x9c2xe2x80x9d and xe2x80x9c3xe2x80x9d recorded with external magnetic fields which have strengths of H0, H1, H2 and H3, respectively, applied to the magneto-optical recording medium in accordance with the recording principle explained in the embodiments. As for information recorded on the basis of the four combinations of states of magnetization, the magneto-optical recording medium is irradiated with reproducing light beams at xcex1=443 nm and xcex2=780 nm to obtain reflected light beams therefrom so that the magnitudes of apparent Kerr rotation angles are determined as reproduced signals.
The term xe2x80x9capparent Kerr rotation anglexe2x80x9d herein means a Kerr rotation angle detected from a reflected light beam from a magneto-optical recording medium irradiated with a reproducing light beam. The apparent Kerr rotation angle is detected as a value which is larger than that of a Kerr rotation angle representing an actual state of magnetization of a magnetic layer due to a multiple interference effect on a protective layer in the recording medium or a Faraday effect of a recording layer. This angle is also called xe2x80x9ceffective Kerr rotation anglexe2x80x9d. FIG. 3(b) shows the relative signal outputs from the respective states reproduced with the reproducing light beam at xcex1=443 nm. FIG. 3(c) shows the relative signal outputs from the respective states reproduced with the reproducing light beam at xcex2=780 nm. As for the magneto-optical recording medium used in the method for recording and reproduction according to the present invention, the apparent Kerr rotation angle obtained with the reproducing light beam varies depending on the wavelength of the reproducing light beam especially due to the multiple interference of the reproducing light beam in the first dielectric layer. As shown in FIG. 2, reproduced signal strengths for the four states of magnetization xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d, xe2x80x9c2xe2x80x9d and xe2x80x9c3xe2x80x9d determined by the combination of the magnetic layers, i.e., apparent Kerr rotation angles, vary depending on the wavelength of the reproducing light beam. In FIG. 2, the difference F in the Kerr rotation angle between the states xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d, the difference E in the Kerr rotation angle between the states xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d and the difference D in the Kerr rotation angle between the states xe2x80x9c3xe2x80x9d and xe2x80x9c2xe2x80x9d in the vicinity of a wavelength xcex=630 nm are approximately the same. However, in a longer wavelength region, for example, the difference A in the Kerr rotation angle between the states xe2x80x9c2xe2x80x9d and xe2x80x9c3xe2x80x9d and the difference B in the Kerr rotation angle between the states xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d at a wavelength xcex=780 nm are fairly larger than the associated differences D and F in the Kerr rotation angle at the wavelength xcex=630 nm. On a shorter wavelength side, the difference C in the Kerr rotation angle between the states xe2x80x9c3xe2x80x9d and xe2x80x9c0xe2x80x9d especially in the vicinity of 443 nm is fairly larger than the associated difference E in the Kerr rotation angle in the vicinity of the wavelength xcex=630 nm. According to these facts, it is recognizable that a higher S/N ratio is obtained by distinguishing and detecting the four states by using the two wavelengths of 443 nm and 780 nm, as compared with detection by dividing signal amplitudes corresponding to the four values of the states xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d, xe2x80x9c2xe2x80x9d and xe2x80x9c3xe2x80x9d by using the single wavelength of 630 nm. According to this embodiment, a reproduced signal is detected by using the reproducing light beam at xcex1=443 nm, and then it is sliced at an appropriate level to make separation into two values concerning the state xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d and the state xe2x80x9c2xe2x80x9d or xe2x80x9c3xe2x80x9d, while a reproduced signal is detected by using the reproducing light beam at xcex2=780 nm, and then it is sliced at appropriate levels to make separation into three levels of the state xe2x80x9c0xe2x80x9d or xe2x80x9c3xe2x80x9d, the state xe2x80x9c1xe2x80x9d, and the state xe2x80x9c2xe2x80x9d so that two series of two-valued signals are obtained, from which a recording signal recorded on the multiple layers (two layers) can be reproduced.
Disclosed as a second reproducing method of the multi-wavelength magneto-optical reproducing method is a method for recording on and reproduction from a magneto-optical recording medium including a plurality of magnetic layers, in which multi-valued data or a plurality of series of two-valued data are recorded on the magneto-optical recording medium as combinations of states of magnetization in the magnetic layers, and the recorded data are reproduced on the basis of the sum of the states of magnetization in the magnetic layers. This reproducing method is characterized in that the magnetic layers are irradiated with light beams having wavelengths xcex1 and xcex2 respectively, the data recorded on one of the magnetic layers are reproduced by using the light beam having the wavelength xcex1, the data recorded on another of the magnetic layers are reproduced by using the light beam having the wavelength xcex2 (xcex2xe2x89xa0xcex1) and thus the data are independently reproduced from each of the magnetic layers. In this reproducing method, the magneto-optical recording medium is adjusted in such a manner that the order or sequence of strength of the reproduced signals detected for the states of magnetization determined by the combination of the states of magnetization differs between the detection at the wavelength xcex1 and the detection at the wavelength xcex2.
In the second reproducing method of the multi-wavelength magneto-optical reproducing method, it is preferred to use a magneto-optical recording medium including two magnetic layers capable of four-valued recording on the basis of four combinations of states of magnetization, in which the order of strength of the signals xcex81 to xcex84 reproduced from the four combinations of states of magnetization differs between the detection at the wavelength xcex1 and the detection at the wavelength xcex2, wherein the two-valued data on one of the magnetic layers are reproduced by using the light beam having the wavelength xcex1, and the two-valued data on the other magnetic layer are reproduced by using the light beam having the wavelength xcex2. In this case, a two-valued signal converted into two-valued one by slicing, at a predetermined level, a reproduced signal including the four combinations of states of magnetization detected at the wavelength xcex1 may correspond to a two-valued state of magnetization of one of the magnetic layers, and a two-valued signal converted into two-valued one by slicing, at a predetermined level, a reproduced signal including the four combinations of states of magnetization detected at the wavelength xcex2 may correspond to a two-valued state of magnetization of the other magnetic layer. The principle of the recording and reproducing method of the multi-wavelength magneto-optical reproducing method will be explained briefly with reference to FIGS. 5 and 6. FIG. 6(a) illustrates states of magnetization recorded on the first and second magnetic layers of a magneto-optical disk produced in Embodiment A2. Four combinations of states of magnetization (↑↑), (↑↓), (↓↑) and (↓↓) exist on the magneto-optical recording medium based on the combinations of directions of magnetization on the first and second magnetic layers. The four combinations of states of magnetization correspond to recorded four-valued recording signals xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d, xe2x80x9c2xe2x80x9d and xe2x80x9c3xe2x80x9d, which have been recorded by applying external magnetic fields having strengths of H0, H1, H2 and H3, respectively, to the magneto-optical recording medium. These signals can be recorded by the foregoing magneto-optical multi-valued recording method. In the second reproducing method of the multi-wavelength magneto-optical reproducing method, the data recorded on the first magnetic layer and the two-valued data recorded on the second magnetic layer can be independently determined (or found) by using the reproducing light beam at xcex1=443 nm and the reproducing light beam at xcex2=780 nm respectively.
The magneto-optical recording medium used for the second reproducing method of the multi-wavelength magneto-optical reproducing method has a characteristic, as shown in FIG. 5, that a curve which represents variation in the apparent Kerr rotation angle with respect to the wavelength concerning one combination of states of magnetization in the two magnetic layers) intersects a curve which represents variation in the apparent Kerr rotation angle with respect to the wavelength concerning another combination of states of magnetization, in a range between the two reproducing wavelengths xcex1 and xcex2 (a curve for the state xe2x80x9c0xe2x80x9d and a curve for the state xe2x80x9c3xe2x80x9d in FIG. 5) . This characteristic can be specifically achieved by adjusting the optical lengths (thicknesses and refractive indexes) of the first dielectric layer and the magnetic layers with respect to the reproducing wavelengths xcex1 and xcex2. FIGS. 6(b) and 6(c) illustrate the relative signal outputs concerning the respective combinations of states of magnetization reproduced by using the light beams at xcex1 and xcex2. As for this medium, the levels (apparent Kerr rotation angles) of the reproduced signals concerning the states xe2x80x9c0xe2x80x9d and xe2x80x9c3xe2x80x9d intersect near 630 nm (FIG. 5). Accordingly, the order of magnitude of the reproduced signals for the states xe2x80x9c0xe2x80x9d and xe2x80x9c3xe2x80x9d is different between xcex2=780 nm and xcex1=443 nm. Specifically, the signal output decreases in an order of xe2x80x9c2xe2x80x9d, xe2x80x9c3xe2x80x9d, xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d upon reproduction with the reproducing light beam at xcex1=443 nm, while the signal output decreases in an order of xe2x80x9c2xe2x80x9d, xe2x80x9c0xe2x80x9d, xe2x80x9c3xe2x80x9d, xe2x80x9c1xe2x80x9d upon reproduction with the reproducing light beam at xcex2=780 nm. Now when the reproduced signal is sliced at an intermediate level of the reproduced signal output at xcex2=443 nm shown in FIG. 6(b), then it is possible to distinguish a set of the two states xe2x80x9c2xe2x80x9d and xe2x80x9c3xe2x80x9d from a set of the two states xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d. According to inspection of the states of magnetization of the two sets of the states, it is recognizable that the set of the two states of xe2x80x9c2xe2x80x9d and xe2x80x9c3xe2x80x9d and the set of the two states of xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d can be distinguished into two-valued information on the basis of the states of magnetization in the first magnetic layer. Specifically, both of the states xe2x80x9c2xe2x80x9d and xe2x80x9c3xe2x80x9d provide a state of magnetization ↓ of the first magnetic layer, while both of the states xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d provide a state of magnetization ↑ of the first magnetic layer. Therefore, a two-valued signal obtained by slicing the reproduced signal at the intermediate level of the reproduced signal output at xcex1=443 nm can correspond to two-valued states of magnetization in the first magnetic layer. On the other hand, when the reproduced signal is sliced at an intermediate level of the reproduced signal output at xcex2=780 nm shown in FIG. 6(c), then the set of the two states of xe2x80x9c2xe2x80x9d and xe2x80x9c0xe2x80x9d is distinguished from the set of the two states of xe2x80x9c3xe2x80x9d and xe2x80x9c1xe2x80x9d, and they can be recognized by using two-valued information. According to inspection of the states of magnetization of the set of the states xe2x80x9c2xe2x80x9d and xe2x80x9c0xe2x80x9d and the set of states xe2x80x9c3xe2x80x9d and xe2x80x9c1xe2x80x9d, both of the states of magnetization in the second magnetic layer are ↑ in the former set, while both of the states of magnetization of the second magnetic layer are ↓ in the latter set. Therefore, a two-valued signal obtained by slicing the reproduced signal at the intermediate level of the reproduced signal output at xcex2=780 nm can correspond to two-valued states of magnetization in the second magnetic layer. Therefore, the use of the magneto-optical recording medium provided with the reproduced signal characteristic as shown in FIGS. 6(b) and (c) makes it possible to independently reproduce two-valued information recorded on the first magnetic layer and two-valued information recorded on the second magnetic layer by selecting the wavelength xcex1 or xcex2 of the reproducing light beam. In this case, it is unnecessary for a focal point of a radiated laser beam to be adjusted at a magnetic layer intended to perform reproduction therefrom.
The reproducing wavelength is not specifically limited in the multi-wavelength magneto-optical reproducing method. However, the reproducing wavelength is preferably xcex1=350 to 900 nm because it is within a wavelength region capable of emission by using currently available various laser apparatuses or capable of emission by using a combination with an SHG (secondary higher harmonic wave generation) device. xcex2 is desirably different in wavelength from xcex1 by not less than 50 nm in order to separate pieces of information recorded on the respective magnetic layers at a high S/N ratio. In the recording and reproducing method described above, the light beams at xcex1 and xcex2 can be radiated so that they are condensed at different portions of a recording area on the magneto-optical recording medium.
In the multi-wavelength magneto-optical reproducing method, the light beam having the wavelength xcex1 or xcex2 is radiated to reproduce information recorded on one of the magnetic layers (recording layers), while the reproduced information and information to be recorded on another of the magnetic layers are recorded in combination with each other so that only information on the another of the magnetic layers may be rewritten in a virtual manner. In the foregoing second recording and reproducing method, information recorded on each of the magnetic layers can be independently reproduced by using a different wavelength for each of the magnetic layers. Therefore, when only one magnetic layer is subjected to rewriting, then information on another magnetic layer not subjected to rewriting is previously reproduced by allowing a reproducing light beam to precede a recording light beam in order to perform scanning on a recording track, and a magnetic field modulation signal is formed by combining the reproduced information with information to be recorded on the magnetic layer subjected to rewriting so that the two magnetic layers are heated by the recording light beam to perform recording while applying a magnetic field in conformity with the modulation signal. Thus, only one magnetic layer is consequently subjected to rewriting.
Suitable for use with the first reproducing method of the multi-wavelength magneto-optical reproducing method is a magneto-optical recording medium in which a ratio of magnitudes of Apparent Kerr rotation angles read from a plurality of states of magnetization determined by the combination of the states of magnetization, obtained upon reproduction by using a light beam having a wavelength xcex1 is mutually different from that obtained upon reproduction by using a light beam having a wavelength xcex2. It is preferred in this magneto-optical recording medium that optical path lengths of layers for constructing the magneto-optical recording medium are adjusted so that the ratio of magnitudes of Apparent Kerr rotation angles read from a plurality of states of magnetization determined by the combination of the states of magnetization, obtained upon reproduction by using the light beam having the wavelength xcex1, is mutually different from that obtained upon reproduction by using the light beam having the wavelength xcex2.
A magneto-optical recording medium for use with the second reproducing method of the multi-wavelength magneto-optical reproducing method includes a plurality of magnetic layers on a substrate. Multi-valued data or a plurality of series of two-valued data are recorded on the medium based on combinations of states of magnetization in the magnetic layers. Suitable as such a recording medium is a magneto-optical recording medium in which magnitudes of apparent Kerr rotation angles read from a plurality of states of magnetization determined by the combination of the states of magnetization differ depending on a wavelength of a reproducing light beam respectively, and which has a magneto-optical characteristic that a curve representing variation in the apparent Kerr rotation angle with respect to the wavelength of the reproducing light beam detected from one combination of states of magnetization intersects a curve representing variation in the apparent Kerr rotation angle with respect to the wavelength of the reproducing light beam detected from at least one other combination of states of magnetization, in a wavelength range between the wavelengths xcex1 and xcex2 of the reproducing light beam.
It is preferable that this magneto-optical recording medium should include at least one dielectric layer and a plurality of magnetic layers on a substrate, and that optical path lengths of the dielectric layer and the magnetic layers be adjusted so that the recording medium has the magneto-optical characteristic that the curve which represents variation in the apparent Kerr rotation angle with respect to the wavelength of the reproducing light beam detected from one combination of states of magnetization intersects the curve which represents variation in the apparent Kerr rotation angle with respect to the wavelength of the reproducing light beam detected from at least one other combination of states of magnetization, in the wavelength range of xcex1 to xcex2 of the wavelength of the reproducing light beam.
A magneto-optical recording medium for use with the multi-wavelength reproducing method includes at least two recording (magnetic) layers, and may also include a first dielectric layer, a second dielectric layer, an auxiliary magnetic layer, a third dielectric layer, a metallic reflective layer and a protective layer between, over or under the recording layers. By stacking these layers in various orders on a transparent substrate, it is possible to form magneto-optical recording media.
The first dielectric layer is provided as a layer for making multiple interference of the reproducing light beam, and increasing the apparent Kerr rotation angle, and it is generally formed of an inorganic dielectric material having a refractive index larger than that of the transparent substrate. Those preferred for the first dielectric layer include, for example, oxides or nitrides of silicon, aluminum, zirconium, titanium, and tantalum, and those especially preferred include SiN. In particular, in the multi-wavelength magneto-optical reproducing method, it is important to control the optical length of the first dielectric layer, i.e., the refractive index and the thickness of the first dielectric layer. If the first dielectric layer is composed of SiN, it should preferably have a refractive index of 1.90 to 2.40. The first dielectric layer composed of SiN having the refractive index as described above can be obtained by adjusting the mixing ratio of sputtering atmosphere gases in conformity with a composition of SiNx upon production of SiN by means of a dry process such as a sputtering method. The first dielectric layer should preferably have a thickness of 400 to 1,100 A (angstroms).
The magnetic (recording and reproducing) layers of a medium for use with the multi-wavelength magneto-optical reproducing method may be composed of an amorphous vertically magnetizable film comprising an alloy of a rare earth metal-transition metal system. Especially, when recording is performed in accordance with the magnetic field modulation system, the magnetic layers should preferably be composed of a material represented by the following general formula:
(Tb100-AQA)XFe100-X-Y-ZCoYMZ
wherein:
15 atomic %xe2x89xa6Xxe2x89xa640 atomic %;
5 atomic %xe2x89xa6Yxe2x89xa620 atomic %;
0 atomic %xe2x89xa6Zxe2x89xa615 atomic %;
0 atomic %xe2x89xa6Axe2x89xa630 atomic %;
wherein: M is at least one element selected from the group consisting of Nb, Cr, Pt, Ti, and Al, and Q is at least one element selected from the group consisting of Gd, Nd, and Dy.
When the recording layer comprises two magnetic layers, both of the first and second magnetic layers may be composed of the alloy of the rare earth metal-transition metal system having the composition described above. The first magnetic layer should preferably have a thickness of 20 to 200 A (asgstroms), and the second magnetic layer should preferably have a thickness of 50 to 500 A (angstroms). The first and second magnetic layers may have various combinations of their Curie temperatures and thicknesses. However, the first and second magnetic layers should preferably have their Curie temperatures which are approximate to one another as close as possible in order to provide a uniform size of magnetic domains recorded on each of the layers. It is preferred that the difference in Curie temperature between the layers is within 30xc2x0 C. It is optimum that the layers have an equal Curie temperature. It is also possible to form a film obtained by stacking three or more magnetic layers.
The auxiliary magnetic layer is added to the magnetic layer for recording so that the auxiliary magnetic layer serves to control an external condition under which recording (i.e., inversion of magnetization) occurs on the magnetic layer. The auxiliary magnetic layer is provided, for example, for controlling a region of an external magnetic field to generate a state of magnetization of each direction (↑or ↓) during recording so that combinations of states of magnetization to be generated on the stacked magnetic layers for recording are generated under different recording conditions. The auxiliary magnetic layer may be composed of, for example, an amorphous vertically magnetizable film of a rare earth-transition metal system, an alloy thin film composed of at least one element selected from the group consisting of noble metals such as Pt, Al, Ag, Au, Cu, and Rh, and at least one element selected from the group consisting of transition metals such as Fe, Co, and Ni, or a film composed of a simple substance of a transition metal such as Fe, Co, and Ni or an alloy film thereof. The auxiliary magnetic layer may have a thickness of 5 to 1,500 A (angstroms).
The metallic reflective layer reflects the reproducing light beam having passed through various layers to return the light beam toward the transparent substrate so that the apparent Kerr rotation angle may be increased owing to the Faraday effect exerted during transmission through the magnetic layer. The metallic reflective layer should preferably be composed of an alloy comprising at least one metal element selected from the group consisting of Al, Ag, Au, Cu, and Be and at least one metal element selected from the group consisting of Cr, Ti, Ta, Sn, Si, Pe, Nb, Mo, Li, Mg, W, and Zr.
The second and third dielectric layers make multiple interference of the reproducing light beam having passed through various layers so that the Kerr rotation angle may be increased, in the same manner as described for the first dielectric layer. The second and third dielectric layers may be composed of the group of materials which may be used to construct the first dielectric layer. In addition, the dielectric layers and the metallic reflective layer also serve as heat control layers to obtain an appropriate recording power sensitivity or an appropriate recording power margin, and they also serve to protect the recording layer from chemical shock. The second and third dielectric layers and the metallic reflective layer are optional layers, which may be omitted.
The transparent substrate is composed of a transparent resin material including, for example, polycarbonate, polymethyl methacrylate, and epoxy, on which a preformat pattern is formed.
The protective layer is the uppermost layer, which may be composed of, for example, an ultraviolet-curable resin. The magneto-optical recording medium having the structure as described above should preferably be produced by a dry process such as sputtering and vapor deposition.
The thickness of each of the first to third dielectric layers, the first and second magnetic layers, and the auxiliary magnetic layer of the magneto-optical recording medium may be appropriately adjusted in accordance with the way of change of the apparent Kerr rotation angle depending on the reproducing wavelength, the apparent Kerr rotation angle being obtained on the basis of the states of magnetization determined by the combination of states of magnetization in the magnetic layers. In particular, the thickness of each of the layers is adjusted so that the ratio and/or the order of magnitude of apparent Kerr rotation angles obtained on the basis of a plurality of combinations of states of magnetization is different between the selected two reproducing wavelengths.
The optical modulation domain magnifying and reproducing method will be explained below. The magneto-optical recording media which can be used with this method can be classified into the following two types of magneto-optical layer structure. For convenience of explanation, the illustrated magneto-optical layer structure includes one recording layer and one reproducing layer. However, the present invention includes a structure in which two or more such layer structures are stacked or laminated.
The first type of magneto-optical layer structure, as illustrated in FIGS. 21A and 21B, includes a first auxiliary magnetic film 5 and a second auxiliary magnetic film 4 sequentially laminated on a magneto-optical recording film 6. The magneto-optical recording film 6, the first auxiliary magnetic film 5 and the second auxiliary magnetic film 4 have magnetic characteristics such that, when the Curie temperatures of the magneto-optical recording film, the first auxiliary magnetic film and the second auxiliary magnetic film are designated as TC0, TC1 and TC2, respectively, and the critical temperatures of the first auxiliary magnetic film and the second auxiliary magnetic film are designated as TCR1 and TCR2, respectively, a relationship expressed as room temperature less than TCR2xe2x89xa6TCR1xe2x89xa6TC0, TC1, TC2 is satisfied.
The first auxiliary magnetic film 5 and the second auxiliary magnetic film 4, as shown in FIG. 22, have magnetic characteristics such that they are plane-magnetized films from room temperature to a certain critical temperature (TCR) higher than room temperature, but are perpendicular-magnetized films at a temperature above TCR. The magneto-optical recording film 6 is a perpendicular-magnetized film at or above room temperature.
The principle of action (reproduction) of the first type of magneto-optical layer structure will be described below. FIG. 21A shows states of magnetization in the layers after writing recording magnetic domains into the magneto-optical recording film 6 by the optical modulation recording system or the like, but before reproducing them. When this medium is irradiated with reproducing light of a suitable power for making the peak temperatures of the magnetic films desired temperatures, a magnetic domain 22 of perpendicular magnetization in the magneto-optical recording film 6 is first transferred to an area in the first auxiliary magnetic film 5 where the temperature has become higher than TCR1. For this purpose, in view of a temperature profile within the medium developed upon radiation with reproducing light as shown in FIG. 27, the reproducing power and TCR1 are set so that a magnetic domain 21 of the same size as, or a smaller size than, the size of the magnetic domain in the magneto-optical recording film 6 will be transferred to the first auxiliary magnetic film 5.
Then, the magnetic domain 21 transferred to the first auxiliary magnetic film 5 is transferred to the second auxiliary magnetic film 4. The critical temperatures of the first and second auxiliary magnetic films are set to satisfy TCR2 less than TCR1. Thus, as indicated by the temperature profile within the medium of FIG. 27, an area in the second auxiliary magnetic film which can be perpendicularly magnetized is larger in diameter than that in the first auxiliary magnetic film. As shown in FIG. 21B, therefore, a transferred magnetic domain 23 in the second auxiliary magnetic film 4 is enlarged or magnified by perpendicular magnetic anisotropy within the perpendicularly magnetizable area in the second auxiliary magnetic film and exchange coupling force resulting from the perpendicular magnetization in the first auxiliary magnetic film 5. This magnetic domain enlargement can be said to be promoted, since the in-plane magnetization in areas indicated at W of the first auxiliary magnetic film 5 in FIG. 21B weakens the exchange coupling force from magnetic domains S of the magneto-optical recording film 6 to the second auxiliary magnetic film 4. This magnetic domain enlargement curtails the decrease in the quantity of light contributing to a reproduction output due to magnetic masking by in-plane magnetization, thus permitting reproduction at a high C/N ratio.
The effect of enlargement of the magnetic domain 23 in the second auxiliary magnetic film 4 becomes maximal when the transferred magnetic domain in the second auxiliary magnetic film 4 is enlarged to a size larger than the diameter of a reproducing light spot. In this state, a very large reproduction output is obtained which is unrelated to the size or shape of the magnetic domain recorded in the magneto-optical recording film 6 and which is determined only by the figure of merit of the second auxiliary magnetic film 4 and the intensity of reproducing light beam. After reproduction, namely, after the reproducing laser light-irradiated area has moved, the readout area is cooled to TCR2 or lower, whereupon the first and second auxiliary magnetic films are returned to a plane-magnetized state, the state of FIG. 21A. Even at temperatures during the reproducing action as described above, the coercivity of the magneto-optical recording film 6 is sufficiently high, so that the information recorded as magnetization is completely retained.
The second type of magneto-optical layer structure, as illustrated in FIG. 26, is characterized in that it includes a nonmagnetic film 9 between an auxiliary magnetic film 8 and a magneto-optical recording film 10, and that the magneto-optical recording film 10 and the auxiliary magnetic film 8 have magnetic characteristics such that when the Curie temperatures of the magneto-optical recording film and the auxiliary magnetic film are designated as TC0 and TC, respectively, and the critical temperature of the auxiliary magnetic film is designated as TCR, a relationship expressed as room temperature less than TCRxe2x89xa6TC0, TC is satisfied.
The principle of reproduction from the second type of magneto-optical layer structure will be described below.
FIG. 25A schematically shows states of magnetization in the auxiliary magnetic film 8, nonmagnetic film 9 and magneto-optical recording film 10 after writing recording magnetic domains into the magneto-optical recording film 10 of the layer structure shown in FIG. 26 by the optical modulation recording system or the like, but before reproducing them. When this magneto-optical layer structure is irradiated with reproducing light of a suitable power for making the peak temperatures of the magnetic films the desired temperatures, an area which can reach TCR or a higher temperature and can be perpendicularly magnetized occurs in the auxiliary magnetic film 8. The TCR and the reproducing power are set so that the size of this area will become not smaller than the diameter of a magnetic domain M recorded in the magneto-optical recording film 10, preferably not smaller than the diameter of a reproducing light spot. The auxiliary magnetic film 8 has magnetic characteristics such that its coercivity has a distribution as shown in FIG. 28 in correspondence with a temperature distribution in the area at or above TCR, and the values of the coercivity are sufficiently small in a region reaching the peak temperature and in the vicinity of the region.
The magneto-optical recording film 10 has a distribution of magnetization as shown in FIG. 28 in correspondence with the temperature distribution in the area above TCR, and a magnetic characteristic such that the values of magnetization are sufficiently large in a region reaching the peak temperature and in the vicinity of the region. Since the magnetic characteristics of the magnetic films have been set as describe above, only the magnetic domain M in the high-temperature, sufficiently high magnetization region in the magneto-optical recording film 10 is transferred to the high-temperature, sufficiently low coercivity region in the auxiliary magnetic film 8 because of a great static magnetic coupling force between the magneto-optical recording film 10 and the auxiliary magnetic film 8 that acts in the region of the magnetic domain M. As a result, a sufficient reproduction resolving power is obtained.
It is conceivable that, as shown in FIG. 25B, a magnetic domain 63 transferred to the auxiliary magnetic film 8 is then enlarged by perpendicular magnetic anisotropy within the region above TCR and exchange coupling force from the transferred magnetic domain. This magnetic domain enlargement strengthens reproduced signals and improves the C/N ratio, as with the first type of magneto-optical recording medium. After reproduction, namely, after the reproducing laser light has moved, the readout area is cooled to TCR or lower, whereupon the auxiliary magnetic film 8 becomes a plane-magnetized film, and returns to the state of FIG. 25A.
The optical modulation domain magnifying and reproducing method involves reproducing a signal by irradiating the first or second magneto-optical recording medium with reproducing light power-modulated at a period which is equal to, or an integral number of times as long as, the period of a reproducing clock. The principle of this reproducing method will be described with reference to FIG. 30, which is a schematic view of the reproducing method. In this reproducing method, the second type of magneto-optical recording medium shown in FIG. 25 is used. First, a predetermined record pattern as shown in FIG. 30(a) is recorded in the magneto-optical recording medium by the optical modulation recording system or the like. In the figure, record marks are recorded with the shortest mark pitch DP, and a record mark length DL is set such that DL=DP/2. During reproduction, the recording medium is irradiated with pulsed laser light, as reproducing laser light, modulated to two reproducing powers Pr2 and Pr1 so that, as shown in FIG. 30(b), the pulsed light will have a period DP synchronized with the record mark positions, and the emission width of the higher power Pr2 will be DL. Light with the lower reproducing power Pr1 is radiated always in an erase state (at sites without record marks), while light with the higher reproducing power Pr2 is radiated always in a record state (at sites with record marks) and an erase state.
FIG. 30(c) shows a signal waveform reproduced by irradiation with reproducing pulsed laser light as shown in FIG. 30(b). FIG. 30(d), on the other hand, shows a signal waveform reproduced from the same track with continuous light having a constant reproducing light power. Pr2 is selected to be a power which will bring about the magnetic domain enlargement in the auxiliary magnetic film 8, as will be described later on, while Pr1 is selected to be a power which will cause the magnetic domain enlargement to vanish. By so selecting the reproducing power, the amplitude Hpl between the record state and the erase state observed with pulsed light reproduction can be set to have the relation Hpl greater than Hdc, where Hdc is the amplitude with reproduction using constant laser light. Furthermore, magnetized information recorded in each magnetic domain of the magneto-optical recording film can be reproduced in an enlarged form independently of, and without influence from, adjacent magnetic domains.
As the optical modulation domain magnifying and reproducing method, the following reproducing method, too, is effective. Used with this method is a magneto-optical recording medium comprising a magneto-optical recording film having perpendicular magnetization, an auxiliary magnetic film which transfers from an in-plane magnetizable film to a perpendicularly magnetizable film when its temperature exceeds a critical temperature Tcr, and a non-magnetic film interposed between the magneto-optical recording film and the auxiliary magnetic film. The magneto-optical recording medium has a magnetic characteristic which satisfies a relationship of room temperaturexe2x89xa6Tcrxe2x89xa6Tcompxe2x89xa6Tcoxe2x89xa6Tc concerning a Curie temperature Tco of the magneto-optical recording film and a Curie temperature Tc and a compensation temperature Tcomp of the auxiliary magnetic film. It is possible to reproduce a recorded signal by irradiating the magneto-optical recording medium with a reproducing light beam power-modulated to have at least two light powers of Pr1 and Pr2 at a period which is equal to, or an integral number of times as long as, the period of a reproducing clock while applying a DC magnetic field to the medium so that a recording magnetic domain in the magneto-optical recording film is transferred to the auxiliary magnetic film, the transferred magnetic domain is magnified, and the magnified magnetic domain is reduced or extinguished. In this method, the light power Pr1 of the reproducing light beam may be a power to heat the auxiliary magnetic film to a temperature from Tcr to Tcomp so that the recording magnetic domain in the magneto-optical recording film is transferred to the auxiliary magnetic film and the magnetic domain is magnified, while the light power Pr2 may be a power to heat the auxiliary magnetic film to a temperature from Tcomp to Tco so that the magnified magnetic domain is reduced or extinguished. It is preferable that such a magneto-optical recording medium is used with this reproducing method that, under a condition in which an external magnetic field Hex is applied to the recording medium, a temperature curve A of a transfer magnetic field generated by the external magnetic field Hex and the magneto-optical recording film and a temperature curve B of a coercive force of the auxiliary magnetic film in a perpendicular direction intersect between room temperature and the compensation temperature Tcomp of the auxiliary magnetic film, and the temperature curves A and B intersect between the compensation temperature Tcomp of the auxiliary magnetic film and the Curie temperature Tco of the magneto-optical recording film. The transfer magnetic field is the sum of the external magnetic field Hex and a static magnetic field Ht from the magneto-optical recording film. The coercive force of the auxiliary magnetic film in the perpendicular direction may be represented by the sum of a coercive force Hr in the perpendicular direction of the magnetic domain subjected to the transfer and an exchange coupling force Hw exerted on the magnetic domain subjected to the transfer by adjoining magnetic domains.
The method for transferring a recording magnetic domain inscribed on the recording layer to the reproducing layer so that the transfer signal on the reproducing layer is magnified and read in order to obtain a high reproduced signal has been confirmed by using the MAMMOS described later. The MAMMOS involves magnifying and reducing a magnetic domain transferred to the reproducing layer with an alternating magnetic field during reproduction. This optical modulation domain magnifying and reproducing method makes it possible to magnify and reduce a transferred magnetic domain by making modulation to give two or more reproducing light powers by using a direct current magnetic field. The principle of reproduction by this reproducing method will be explained. The reproducing method is based on the use of a magneto-optical recording medium comprising a magneto-optical recording film having perpendicular magnetization, an auxiliary magnetic film which causes transition from an in-plane magnetizable film to a perpendicularly magnetizable film when its temperature exceeds a critical temperature Tcr, and a non-magnetic film interposed therebetween. FIG. 37 shows a magneto-optical recording medium of this type. The magneto-optical disk 90 shown in FIG. 37 comprises, in a stacked form on a substrate 1, a dielectric film 3, an auxiliary magnetic film 8, a non-magnetic film 3, a magneto-optical recording film 10, and a protective film 7. The auxiliary magnetic film 8 has a compensation temperature Tcomp between a critical temperature Tcr and its Curie temperature Tc. The magneto-optical recording medium 90 satisfies the relationship of room temperature less than Tcr less than Tcomp less than Tco less than Tc concerning the Curie temperature Tco of the magneto-optical recording film 10, the critical temperature Tcr, the Curie temperature Tc, and the compensation temperature Tcomp of the auxiliary magnetic film 8.
Reproduction is performed in accordance with the optical modulation domain magnifying and reproducing method by radiating the light power-modulated reproducing light beam while applying the external DC magnetic field to the magneto-optical recording medium 90 having the magnetic characteristic as described above. FIG. 39 shows magnetic characteristics of the magneto-optical recording film 10 and the auxiliary magnetic film 8 of the magneto-optical disk 90 in a state in which the constant DC magnetic field Hex is applied to the magneto-optical recording medium 90 in the recording direction. The magnetic temperature curve A shown in FIG. 39 denotes a temperature-dependent change in transfer magnetic field (static magnetic field) generated by the magnetization of the recording layer from the magneto-optical recording film 10 (hereinafter simply referred to as xe2x80x9crecording layerxe2x80x9d) to the auxiliary magnetic film 8 (hereinafter simply referred to as xe2x80x9creproducing layerxe2x80x9d). The transfer magnetic field of the curve A represents the magnitude of the magnetic field obtained by adding an amount of offset of the external magnetic field Hex. Therefore, the magnetic filed having the magnitude of (Hexxe2x88x92Ht) and the magnetic field having the magnitude of (Hex+Ht) exist as the entire transfer magnetic field depending on the direction of the magnetic domain of the recording layer, with a boundary of the Curie temperature Tco of the recording layer. The two magnetic fields constitute the curve A. In FIG. 39, the downward direction is the recording direction. Hex is applied in the downward direction. In this case, the external magnetic field Hex is adjusted to be small as compared with the magnitude of the static magnetic field Ht in the initializing direction generated from the magnetization of the recording layer at room temperature. Therefore, the entire transfer magnetic field includes those directed in the upward direction (negative) and in the downward direction (positive) depending on the direction of magnetization of the recording magnetic domain in the recording layer as illustrated by the curve A.
The magnetic temperature curve B denotes the temperature-dependent change of the coercive force in the perpendicular direction of the reproducing layer in a state of having the perpendicular magnetization. The coercive force is represented by Hr+Hw as including the pure coercive force Hr of the magnetic domain in the reproducing layer in the perpendicular direction and the magnetic field Hw corresponding to a virtual magnetic field regarded to be applied by generation of the magnetic wall of the reproducing layer (in other words, the exchange coupling magnetic field in the in-plane direction of the reproducing layer). That is, Hr+Hw represents the magnetic field necessary to perform inversion of the magnetization in the direction perpendicular to the film surface of the reproducing layer. As shown in FIG. 39, the magnetization in the direction perpendicular to the film surface of the reproducing layer appears at a temperature which is not less than the critical temperature Tcr (T0 in FIG. 39) at which the reproducing layer behaves as a perpendicularly magnetizable film. The coercive force is maximal at the compensation temperature Tcomp because the magnetization of the reproducing layer is zero.
The temperature curves A and B shown in FIG. 39 are divided into those belonging to three areas (a) to (c) as shown in FIG. 39. The three areas (a) to (c) correspond to the three steps of i) magnetic domain transfer from the recording layer to the reproducing layer, ii) magnification of the transferred magnetic domain in the reproducing layer, and iii) extinguishment of the magnified magnetic domain, respectively, in the reproducing method shown in FIG. 40(a). Accordingly, explanation will be made with reference to FIG. 40 for the magnetic characteristics required for the recording layer and the reproducing layer in the areas (a) to (c) shown in FIG. 39. Arrows in the recording layer and the reproducing layer shown in FIG. 40(a) denote the direction of the magnetic moment of the rare earth metal included in each of the magnetic domains.
The area (a) is a temperature area in which the magnetic domain is transferred from the recording layer to the reproducing layer in this reproducing method, which belongs to a temperature range of T0 to T1 in the figure. T0 means the critical temperature Tcr, and T1 is a temperature at which the magnetic temperature curve A on the side of Hexxe2x88x92Ht initially intersects the magnetic temperature curve B. The temperature range T0 to T1 can be achieved by adjusting the light power of the reproducing light beam to be a relatively low power as described later on. In order to actually perform the magnetic transfer as shown in FIG. 40(a)(1) in this temperature area, it is necessary that the magnitude of the transfer magnetic field in this temperature area exceed the coercive force of the reproducing layer in the perpendicular direction. That is, when the magnetization recorded on the recording layer is in the direction ↓ (recording direction), it is necessary that the transfer magnetic field represented by Hex+Ht is larger than Hr+Hw or xe2x88x92(Hr+Hw) (requirement for magnetic domain transfer). When the magnetization recorded on the recording layer is in the direction ↑ (erasing direction), it is necessary that the negative transfer magnetic field represented by Hexxe2x88x92Ht is smaller than the coercive force Hr+Hw or xe2x88x92(Hr+Hw) of the reproducing layer in the perpendicular direction (requirement for magnetic domain transfer).
On the other hand, when the magnetic temperature curves A and B are compared with each other in the area (a) shown in FIG. 39, it is appreciated that the relationships of the following expressions (a1) to (a3) hold.                     Hr         less than                   Hex          +          Ht          -          Hw                                    (        a1        )                                          -          Hr                 greater than                   Hex          -          Ht          +          Hw                                    (        a2        )                                Hr         greater than                   Hex          -          Ht          -          Hw                                    (        a3        )            
Therefore, the area (a) satisfies the magnetic domain transfer requirement described above, and the recording magnetic domain in the recording layer can be transferred to the reproducing layer regardless of the direction of magnetization thereof. FIG. 40(a)(1) shows a case in which the magnetization in the direction ↓ recorded in a magnetic domain 210 in the recording layer is transferred to an area of the reproducing layer at a temperature which exceeds the temperature T0 within the reproducing light spot so that a transferred magnetic domain 201a is formed.
Subsequently, in the area (b) shown in FIG. 39, the magnetic domain magnification is performed for the magnetic domain 201b transferred to the reproducing layer as shown in FIG. 40 (2) and (3). This temperature area resides in a range indicated by T1 to T2 in the figure. The temperature T2 is a temperature at which the magnetic temperature curve A on the side of Hexxe2x88x92Ht intersects the magnetic temperature curve B on the high temperature side. The magneto-optical disk having the magnetic characteristic shown in FIG. 39 is adjusted such that T2 is approximately coincident with the compensation temperature Tcomp of the reproducing layer (the temperature exists between the compensation temperature Tcomp and the Curie temperature Tco of the recording layer, and the temperature is a temperature extremely close to the compensation temperature Tcomp) in relation to the external magnetic field Hex. In this temperature area, as shown in FIG. 40(a)(2), magnetic domains 203, 203xe2x80x2, which are subjected to magnetic transfer from magnetic domains 212, 212xe2x80x2 in the recording layer in the upward direction, exist on both sides of the magnetic domain 201b transferred to the reproducing layer, as a result of being heated to T0 to T1 within the reproducing light spot. In order to allow the magnetic domain 201b transferred to the reproducing layer to start magnification in the in-plane direction, it is necessary that the directions of the magnetic domains 203, 203xe2x80x2 disposed on the both sides are directed to the recording direction (direction ↓) in the same manner as the magnetic domain 201b. The magnetic domains 203, 203xe2x80x2 receives the transfer magnetic field (Hexxe2x88x92Ht) (totally in the direction ↑) obtained by adding, to the external magnetic field Hex, the static magnetic field Ht in the upward direction from magnetic domains 212 in the recording layer existing just thereover. On the other hand, the magnetic domains 203, 203xe2x80x2 have the coercive force in the perpendicular direction including the exchange coupling magnetic field Hw (in the downward direction) exerted by the magnetic domain 201b and the coercive force Hr to invert the magnetization of the magnetic domains 203, 203xe2x80x2 themselves. Therefore, when the coercive force in the perpendicular direction (Hr+Hw) is made larger than the transfer magnetic field (Hexxe2x88x92Ht) of the magnetic domains. 203, 203xe2x80x2, the magnetic domains 203, 203xe2x80x2 are inverted (requirement for magnetic domain inversion).
It is appreciated that the following relational expressions hold in the area (b) according to the relative magnitude between the magnetic temperature curves A and B.                     Hr         less than                   Hex          +          Ht          -          Hw                                    (        b1        )                                          -          Hr                 less than                   Hex          -          Ht          +          Hw                                    (        b2        )                                Hr         greater than                   Hex          -          Ht          -          Hw                                    (        b3        )            
The foregoing expression (b2) is the condition of magnetic domain inversion itself under which the coercive force (Hr+Hw) in the perpendicular direction is larger than the transfer magnetic field Hexxe2x88x92Ht (in the upward direction) of the magnetic domains 203, 203xe2x80x2. Therefore, the magnetic domain magnification occurs in the area (b) for the magnetic domain 201b in the reproducing layer as shown in FIG. 39(a)(3). According to the relationship of (b2), it is demonstrated that no magnetic domain in the downward direction appears in the reproducing layer when there is no magnetic domain in the recording direction in the reproducing layer, in the temperature area (b). In FIG. 39(a)(3) , the both sides of the magnified magnetic domain 201b are the temperature area of T0 to T1. Therefore, the magnetic domains 203, 203xe2x80x2 in the direction ↑, which are subjected to the magnetic domain transfer from the magnetic domains 212, 212xe2x80x2 in the recording layer, exist therein.
Subsequently, in the area (c), the transferred and magnified magnetic domain is inverted (extinguished), and a magnetic domain 201c in the erasing direction is formed as shown in FIG. 40(a)(4). This temperature area exists in a range from T2 which slightly exceeds the compensation temperature of the reproducing layer, to the Curie temperature Tco of the recording layer. The magnified and reproduced magnetic domain can be extinguished or reduced by applying the reproducing magnetic field in the erasing direction, i.e., by using the alternating magnetic field as the reproducing magnetic field. However, in this reproducing method, the DC magnetic field is used to extinguish the magnified magnetic domain by power-modulating the reproducing light beam to have the power higher than the reproducing light power used to perform the magnetic transfer and the magnification. The reproducing light power may be modulated to be further small in order to extinguish the magnified magnetic domain, as described in Embodiment B2 of the reproducing method on the magneto-optical recording medium as described later on.
Explanation will be made with reference to FIG. 41 for the principle to invert (extinguish) the magnified magnetic domain in the area (c). FIG. 41 illustrates the temperature-dependent change of the direction and the magnitude of sub-lattice magnetization of the rare earth metal and the transition metal of the magnetic domain 210 in the recording layer composed of the rare earth-transition metal (TbFeCo alloy) and the magnetic domain 201b in the reproducing layer composed of the rare earth-transition metal (GdFeCo alloy) subjected to the magnetic domain transfer therefrom shown in FIG. 40(a)(2). As shown in FIG. 41(a), when the temperature of the reproducing layer is less than the compensation temperature Tcomp, then the magnetization of the rare earth metal in the reproducing layer is dominant, and it is parallel to the direction of magnetization of the recording layer of the transfer source (the magnetization of the transition metal is dominant). Subsequently, when the temperature of the reproducing layer exceeds the compensation temperature Tcomp by radiating the high power laser, the magnetic moment of the transition metal in the reproducing layer is dominant. It is appreciated that the following expressions (c1) and (c2) hold according to the relative magnitude of the magnetic temperature curves A and B of the reproducing layer and the recording layer in the area (c) shown in FIG. 39.                     Hr         less than                   Hex          +          Ht          -          Hw                                    (        c1        )                                Hr         less than                   Hex          -          Ht          -          Hw                                    (        c2        )            
That is, the coercive force Hr of the magnetic domain 201b is smaller than the entire magnetic field (Hex+Htxe2x88x92Hw or Hexxe2x88x92Htxe2x88x92Hw) in the recording direction acting on the magnetic domain 201b. As a result, when the temperature of the reproducing layer is not less than the compensation temperature Tcomp (exactly, when it is not less than T2), the dominant magnetic moment of the transition metal is inverted to be directed in the recording direction as shown in FIG. 41(b). Therefore, the magnetic moment of the rare earth metal in the downward direction of the magnified magnetic domain 201b shown in FIG. 40(a)(3) is inverted in the area which is heated to the temperature not less than the temperature of the area (c), i.e., not less than the compensation temperature Tcomp. Thus, the inverted magnetic domain 201c is generated (FIG. 40(a)(4)). The magnetic domains 201d, 201dxe2x80x2, which are disposed on the both sides of the inverted magnetic domain 201c, have their temperatures ranging from T1 to T2. Therefore, the magnetic domains 201d, 201dxe2x80x2 have the same direction of magnetization as that of the magnified magnetic domain 201b. 
In the foregoing reproducing method, the three temperature areas (a) to (c) can be achieved by modulating the reproducing light power to have at least the two power levels Pr1 and Pr2 as shown in FIG. 40(b). That is, the light power Pr1 of the reproducing light beam may be the power for heating the auxiliary magnetic layer to the temperature of Tcr to Tcomp and making it possible to transfer the recording magnetic domain in the magneto-optical recording film to the reproducing layer and magnify the magnetic domain. The light power Pr2 of the reproducing light beam may be the power for heating the auxiliary magnetic layer to the temperature of Tcomp to Tco and reducing or extinguishing the magnified magnetic domain as described above. The PR1/Pr2 power-modulated reproducing light beam is used as the reproducing light beam in synchronization with the reproducing clock. Thus, the recording magnetic domain in the recording layer can be subjected to reproduction through the steps of i) transfer to the reproducing layer, ii) magnification of the transferred magnetic domain, and iii) extinguishment of the magnified magnetic domain.
With reference to FIG. 42, a magneto-optical layer structure suitable for the foregoing reproducing method will be explained. As shown in FIG. 42, a magneto-optical recording medium 100 successively comprises, on a magneto-optical recording film 10, a first auxiliary magnetic film 28, a non-magnetic film 29, and a second auxiliary magnetic film 24. The magneto-optical recording film 10 is a perpendicularly magnetizable film. The first auxiliary magnetic film 28 is a magnetic film which causes transition from a perpendicularly magnetizable film to an in-plane magnetizable film when the temperature exceeds the critical temperature Tcr11. The second auxiliary magnetic film 24 is a magnetic film which causes transition from an in-plane magnetizable film to a perpendicularly magnetizable film when the temperature exceeds the critical temperature Tcr12. It is assumed herein that materials and compositions of the magnetic films are adjusted so that the critical temperature Tcr11 of the first auxiliary magnetic film is higher than the critical temperature Tcr12 of the second auxiliary magnetic film. The second auxiliary magnetic film 24 functions as a reproducing layer.
With reference to FIGS. 44(a) to 44(c), the principle of reproduction from this magneto-optical layer structure will be explained. FIG. 44(a) conceptually illustrates main components of the magneto-optical recording medium shown in FIG. 42. It is assumed that the magnetization in the upward direction is recorded in a magnetic domain 22 of the magneto-optical recording film 10. The magneto-optical recording film 10 and the first auxiliary magnetic layer 28 make exchange coupling to one another. The same magnetization as that of the magnetic domain 22 is transferred to a magnetic domain 28a of the first auxiliary magnetic layer 28 disposed just under the magnetic domain 22. When the magneto-optical recording medium is irradiated with a reproducing light means, and the temperature begins to rise, then the transition occurs from the in-plane magnetization to the perpendicular magnetization in an area of the second auxiliary magnetic film 24 in which its temperature exceeds the critical temperature Tcr12. The area subjected to the transition corresponds to magnetic domains 24a, 24b shown in FIG. 44(b). During the transition, the magnetic domain 24a is aligned in the same direction of magnetization as that of the magnetic domain 22 as shown in FIG. 44(b) by the aid of the magnetostatic coupling force exerted by the magnetic domain 22 of the recording layer 10 disposed just thereover and the magnetic domain 28a of the first auxiliary magnetic film 28. FIG. 44(b) illustrates the temperature-rising process of the magneto-optical recording medium effected by the reproducing light beam, and it represents a state of magnetization in which the temperature T of the magneto-optical recording medium does not arrive at a maximum arrival temperature yet and the temperature is within a range of Tcr12 less than T less than Tcr11. In this state, the recording layer 10, the first auxiliary magnetic layer 28, and the second auxiliary magnetic layer 24 are magnetically coupled (magnetostatically coupled) to one another, and any of them exhibits the perpendicular magnetization. Minute magnetic domains 24b, which have the magnetization in the downward direction by the aid of the magnetostatic coupling force exerted by the both magnetic domains adjacent to the magnetic domain 22 and the magnetic domains in the downward direction in the first auxiliary magnetic film 28 disposed just thereunder, are present on both adjoining sides of the magnetic domain 24a. 
When the temperature of the medium is further raised to arrive at the heating maximum temperature, if the temperature of the high temperature area of the first auxiliary magnetic layer 28 exceeds the critical temperature Tcr11, then the coercive force of the first auxiliary magnetic layer 28 is lowered, and thus the first auxiliary magnetic layer 28 in the high temperature area causes transition from the perpendicular magnetization to the in-plane magnetization. As a result, a magnetic domain 28axe2x80x2 is formed as shown in FIG. 44(c).
FIG. 45 shows a relationship between the temperature distribution and the state of magnetization of the medium shown in FIG. 44(c). In the case of this magneto-optical recording medium, there is given Tcr12 less than Tcr11 as described above. Accordingly, as shown in FIG. 45, the area, in which the temperature exceeds Tcr12 in the temperature distribution of the medium, is wider than the area in which the temperature exceeds Tcr11. The transition occurs from the in-plane magnetization to the perpendicular magnetization in the area in which the temperature exceeds Tcr12 in the second auxiliary magnetic layer 24. The transition occurs from the perpendicular magnetization to the in-plane magnetization in the area in which the temperature exceeds Tcr11 in the first auxiliary magnetic layer 24. Therefore, the magnetic domain 24axe2x80x2 having the perpendicular magnetization in the second auxiliary magnetic layer 24 is larger than the magnetic domain 28a xe2x80x2 having the in-plane magnetization in the first auxiliary magnetic layer 24. The reproducing light power and Tcr12 are adjusted so that the area, in which the temperature exceeds Tcr12 in the second auxiliary magnetic layer 24 upon irradiation with the reproducing light beam, is larger than the magnetic domain in the recording layer 10.
On the other hand, the magnetic domain 28axe2x80x2 in the first auxiliary magnetic layer 28 has the in-plane magnetization. Therefore, the magnetic influence can be intercepted, which would be otherwise exerted from the magneto-optical recording film 10 to the second auxiliary magnetic film 24, due to, for example, the leakage magnetic field and the static magnetic field caused by the magnetization in the direction ↓ existing on both adjoining sides of the magnetic domain 22. Accordingly, it is possible to facilitate the magnification of the magnetic domain 24axe2x80x2. The magnification of the magnetic domain increases the reproduced signal. It is considered that C/N is improved owing to the function of the first auxiliary magnetic film 24 to cause magnetic interception. In order to more effectively use the magnetically intercepting function of the first auxiliary magnetic film 28, it is preferable that the critical temperature Tc11 of the first auxiliary magnetic film 28 and the reproducing light power are selected so that the area, in which the temperature exceeds Tcr11 in the first auxiliary magnetic layer 28 during reproduction, is larger than the recording magnetic domain 11. In order to obtain a sufficiently large reproduced signal by the aid of the magnetic domain magnification in the second auxiliary magnetic layer 24, it is preferable that the critical temperature Tc12 of the second auxiliary magnetic film 24 and the reproducing light power are selected so that the area, in which the temperature exceeds Tcr12 in the second auxiliary magnetic layer 24 during reproduction, is larger than the recording magnetic domain 11. In order to simultaneously satisfy the facilitating effect for magnifying the magnetic domain and the magnetically intercepting function of the first auxiliary magnetic film 28, it is desirable to appropriately control the relationship (xcex94T Tcr11xe2x88x92Tcr12) between the critical temperature Tcr11 of the first auxiliary magnetic film 28 and the critical temperature Tcr12 of the second auxiliary magnetic film 24.
The effect of the magnification of the magnetic domain of the second auxiliary magnetic film 24, i.e., the reproduced signal intensity is maximized when the transferred magnetic domain in the second auxiliary magnetic film 24 is magnified to be not less than the reproducing light spot diameter. In this state, an extremely large reproduction output, which is determined by only the performance index of the second auxiliary magnetic film 24 and the reproducing light beam, is obtained regardless of the size and the shape of the magnetic domain recorded in the magneto-optical recording film 10. After the reproduction, i.e., after the unit for radiating the reproducing light beam is moved, the readout portion is cooled to be not more than Tcr12, and the second auxiliary magnetic film is in the in-plane state of magnetization to return to the state shown in FIG. 44(a). The coercive force of the magneto-optical recording film 10 is sufficiently large even at the temperature during the reproducing operation as described above. Therefore, the information recorded as magnetization is completely retained.
It is desirable for the foregoing magneto-optical layer structure that, as shown in FIG. 43, a relationship of room temperature less than Tcr12 less than Tcr11 less than Tco, Tc1, Tc2 should hold concerning a Curie temperature Tco of the magneto-optical recording film, a Curie temperature Tc1 and the critical temperature Tcr11 of the first auxiliary magnetic film, and a Curie temperature Tc2 and the critical temperature Tcr12 of the second auxiliary magnetic film.
With reference to FIGS. 49 to 53, the principle of the MAMMOS will be explained below. FIG. 49A illustratively shows a concept for recording information as minute magnetic domains on a magneto-optical recording medium 11 suitable for the MAMMOS by applying a recording magnetic field 15 to the medium 11 while irradiating the medium 11 with a recording laser beam 13. The magneto-optical recording medium 11 comprises a magnetic domain-magnifying and reproducing layer 3, an intermediate layer 4, a gate layer 16, an exchange coupling force control layer 17, and an information-recording layer 18. Information can be recorded on the magneto-optical recording medium 11 based on the use of the optical magnetic field modulation system, wherein the magneto-optical recording medium 11 is irradiated with a laser pulse synchronized with a recording clock while applying a magnetic field having a polarity corresponding to a recording signal. The magneto-optical recording medium 11 is moved in a traveling direction indicated by an arrow in FIG. 49A with respect to a recording laser beam 13. Therefore, an area 19, which is deviated backward from the spot center, is heated to a higher temperature. The coercivity of the area 19 in the information-recording layer 18 is lowered due to the heating. Accordingly, a minute magnetic domain, which has a direction of magnetization directed in the direction of the recording magnetic field 15, is formed during its cooling process. It is assumed in the description of the principle that the magneto-optical recording medium is subjected to recording and reproduction by using, for example, a magneto-optical recording and reproducing apparatus 200 conceptually illustrated in FIG. 50. With reference to FIG. 50, the magneto-optical recording medium 210 is rotationally movable with respect to an optical head 213 and a flying magnetic head 215 by the aid of a spindle motor 217, and an initializing magnetic field is applied to the magneto-optical recording medium 210 by the aid of an initializing magnet 211 upon reproduction.
As shown in FIG. 49B, the initializing magnetic field 12 is applied to the magneto-optical recording medium 11, in a direction opposite to the direction of the recording magnetic field 15. The coercivity of the gate layer 16 at room temperature is smaller than the initializing magnetic force. Therefore, the magnetic domains subjected to recording in the gate layer 16 are inverted, and all of them are directed in the direction of the initializing magnetic field 12. On the contrary, the coercivity of the information-recording layer 18 is extremely larger than the coercivity of the gate layer 16. Therefore, magnetization of a recording magnetic domain 313b in the information-recording layer 18 remains as it is. Magnetization of the gate layer 16 is antiparallel to that of the magnetic domain 313b in the information-recording layer 18. Therefore, an interface therebetween is in an unstable state of magnetization.
After the gate layer 16 is initialized as described above, the magneto-optical recording medium 11 is subjected to reproduction under a reproducing light beam as shown in FIG. 51. During reproduction, the magneto-optical recording medium is irradiated with the reproducing light beam having a power lower than that of the recording light beam. An area 314, which is deviated backward from the spot center, is heated to a higher temperature in the same manner as heated by the recording light beam. The coercivity of the gate layer 16, which corresponds to the area 314 heated to the higher temperature, is lowered. The magnetic domain 313b in the information-recording layer 18 is transferred to the gate layer 16 via the exchange force control layer 17 by the aid of the exchange coupling force between the information-recording layer 18 and the gate layer 16, and it is further transferred to the magnetic domain-magnifying and reproducing layer 3. On the other hand, another recording magnetic domain 313a in the information-recording layer 18 is not transferred to the gate layer 16, because an area in the gate layer 16 corresponding to the magnetic domain 313a has a relatively low temperature, and its coercivity is not lowered. Therefore, as shown in a lower part of FIG. 51, when the magneto-optical recording medium 11 is enlarged and viewed from an upward position , only an area 315, which has arrived at a high temperature in the laser spot 311, undergoes decrease in magnetic energy. Accordingly, the recording magnetic domain 313b in the information-recording layer 18 appears as a recording mark 316 on the gate layer 16, and it appears on the magnetic domain-magnifying and reproducing layer 3. On the other hand, the other magnetic domains 313 are prevented from transfer by the gate layer 16, in areas other than the area 315 in the spot 311. Therefore, the recording magnetic domain 313a in the information-recording layer 18 remains latent. Accordingly, it is possible to independently reproduce only one minute magnetic domain of a plurality of minute magnetic domains existing within the spot size, by irradiating the magneto-optical recording medium with the reproducing light beam in accordance with the principle as shown in FIG. 51.
According to the MAMMOS, one minute magnetic domain, which is focused by using the gate layer 16 as described above, can be transferred to the magnetic domain-magnifying and reproducing layer 3, and it can be magnified within the reproducing laser spot. This process is performed in the magnetic domain-magnifying and reproducing layer 3 of the magneto-optical recording medium 11. This principle will be explained with reference to FIG. 52A. It is noted that the magnetic domain-magnifying and reproducing layer 3 is a magnetic layer to which a minute magnetic domain is transferred from the gate layer 16, and on which the transferred magnetic domain can be magnified by the aid of the reproducing magnetic field. The magnetic domain-magnifying and reproducing layer 3 is a perpendicularly magnetizable film having a magnetic force resistance of the magnetic wall which is smaller than the force of the reproducing magnetic field upon being irradiated with the reproducing light beam so that the magnetic wall is moved by application of the reproducing magnetic field to magnify the magnetic domain. When a magnifying reproducing magnetic field 411 is applied in a direction identical with that of magnetization of the minute magnetic domain 313b in the reproducing state shown in FIG. 51, i.e., in the state in which the minute magnetic domain 313b is transferred from the information-recording layer 18 to the gate layer 16 and the magnetic domain-magnifying and reproducing layer 3, then the magnetic wall is moved in a direction to magnify the magnetic domain, because the magnetic force resistance of the magnetic wall is small in the magnetic domain-magnifying and reproducing layer 3. Thus, a magnified magnetic domain 419 is formed. As a result, as shown in a lower part of FIG. 52A, it is possible to observe a magnified mark 413 (the magnetic domain 419 magnified in the magnetic domain-magnifying and reproducing layer) magnified within the reproducing spot 311. As described above, the minute magnetic domain which has been magnified appears on the surface of the magneto-optical recording medium. Therefore, a reproduced signal having a sufficient intensity can be obtained from the magnified magnetic domain.
After the magnified magnetic domain 419 in the information-recording layer 18 is subjected to reproduction, a reducing reproducing magnetic field 415 is applied in a direction opposite to that of the magnifying reproducing magnetic field 411 as shown in FIG. 52B. Accordingly, the magnified magnetic domain 419 in the magnetic domain-magnifying and reproducing layer 3 is reduced. As a result, areas having a direction of magnetization identical with the direction of the magnetic field of the reducing reproducing magnetic field 415 are predominant. The reducing reproducing magnetic field 415 and the magnifying reproducing magnetic field 411 can be applied by using an alternating magnetic field. A reproduced signal with amplification for each of the minute magnetic domains can be obtained by synchronizing the period of the alternating magnetic field with a recording clock.
Now, explanation will be made with reference to a hysteresis curve shown in FIG. 53 for the relationship among the magnitude of the magnifying reproducing magnetic field applied during reproduction, the applied magnetic field, and the size of the mark appearing on the magnetic domain-magnifying and reproducing layer 3. The hysteresis curve shown in FIG. 53 illustrates the change in Kerr rotation angle xcex8K of the magnetic domain-magnifying and reproducing layer 3 with respect to the magnetic field H. The Kerr rotation angle xcex8K is observed when various magnetic fields H are applied to the magneto-optical recording medium while irradiating the magneto-optical recording medium with a reproducing light beam having the same power as that used during reproduction. It is noted that the hysteresis curve shows a hysteresis curve of the magnetic domain-magnifying and reproducing layer of the magneto-optical recording medium having the structure shown in FIGS. 51 and 52, to which the recording magnetic domain in the underlying information-recording layer is transferred by being irradiated with the reproducing light beam. A predetermined Kerr rotation angle xcex8 is provided (point a in the figure/s) even when the magnetic field H is zero, because the magnetic domain in the information-recording layer has been transferred. When the magnetic field H having a polarity identical with the polarity of magnetization of the recording magnetic domain is gradually applied, the initial magnetization curve rises. The point b represents an initial rising point. The rise of the initial magnetization curve corresponds to magnification of the magnetic domain in the layer (the magnetic domain 419 in FIG. 52A) as a result of movement of the magnetic wall of the magnetic domain-magnifying and reproducing layer 3 from the center of the magnetic domain toward the outside depending on the magnitude of the magnetic field H. In the initial magnetization curve, no more increase in Kerr rotation angle occurs when magnetization is saturated. Conceptual photomicrographs of magnetic domain patterns are shown, in which the magnetic domain-magnifying and reproducing layer 3 is viewed from an upward position, at respective points including the points a and b on the initial magnetization curve of the hysteresis curve. The magnetic domain pattern (black circle pattern) at the point a concerns magnetic domains obtained when magnetic domains (seed magnetic domains) in the information-recording layer 18 are transferred via the gate layer 16 to the magnetic domain-magnifying and reproducing layer 3 by the aid of irradiation with the reproducing light beam. The patterns at the respective points comprehensively suggest the situation in which the magnetic domains are magnified in accordance with the increase of the magnetic field on the initial magnetization curve starting from the state represented by the point a. When the Kerr rotation angle xcex8 is saturated, the magnetic domains are inverted on the entire surface of the magnetic domain-magnifying and reproducing layer 3.
In the hysteresis curve shown in FIG. 53, the magnetic field at the rising point c of the major loop of the hysteresis curve (outer loop which represents a locus after the initial magnetization curves is once saturated), which has the same polarity as that of the magnetic field applied in the direction to magnify the magnetization of the magnetic domain-magnifying and reproducing layer, is referred to as xe2x80x9cnucleation magnetic fieldxe2x80x9d. The absolute value thereof is represented by Hn. The magnetic field at the initial rising point b of the initial magnetization curve, which is obtained by applying the magnetic field in the direction to expand the recording magnetic domain in the magnetic domain-magnifying and reproducing layer 3 transferred from the information-recording layer 5 via the gate layer 16, is referred to as xe2x80x9cmagnetic wall-magnifying magnetic fieldxe2x80x9d. The absolute value thereof is represented by He. Assuming that the reproducing magnetic field has its absolute value Hr, it is desirable to apply the reproducing magnetic field within a range of He less than Hr less than Hn because of the following reason. Namely, if Hr is smaller than He, the recording magnetic domain transferred to the magnetic domain-magnifying and reproducing layer 3 is not magnified. If Hr is larger than Hn, even when no recording magnetic domain (seed magnetic domain) exists in the information-recording layer 18, then the magnetic domain in the magnetic domain-magnifying and reproducing layer 3 disposed thereover is inverted, and it is read as a signal.
FIG. 54 shows an initial magnetization curve obtained when the magnetic field is applied in a direction to reduce the recording magnetic domain in the magnetic domain-magnifying and reproducing layer 3 transferred via the gate layer 16 from the information-recording layer 18, in the hysteresis curve shown in FIG. 53. The magnetic field at the initial dropping point cxe2x80x2 of the major loop (outer loop which represents a locus after the initial magnetization curve is once saturated) of the hysteresis curve, which is located on the side of the same polarity as that of the initial magnetization curve, is referred to as xe2x80x9cnucleation magnetic fieldxe2x80x9d. The absolute value thereof is represented by Hn. The magnetic field at the dropping point d on the initial magnetization curve is referred to as xe2x80x9cmagnetic wall-reducing magnetic fieldxe2x80x9d. The absolute value thereof is represented by Hs. When the magnetic field is applied within a range of Hs less than Hr, the magnetic field having been subjected to magnification and reproduction can be reduced. In FIG. 54, conceptual photomicrographs of magnetic domain patterns are also shown, in which the magnetic domain-magnifying and reproducing layer is viewed from an upward position, at respective points including the points a and d on the initial magnetization curve of the hysteresis curve. Since the magnetic field in the reducing direction is too large at the point e, the recording magnetization transferred to the magnetic domain-magnifying and reproducing layer completely disappears. Therefore, when it is intended to reliably erase the recording magnetization, it is appropriate to adjust the magnetic field to satisfy Hs less than Hn less than Hr. The hysteresis curves depicted in FIGS. 53 and 54 and hysteresis curves referred to herein are hysteresis curves obtained under the condition in which magneto-optical reproduction is performed in accordance with the reproducing method for the MAMMOS, and they represent characteristics of the Kerr rotation angle (or magnetization) with respect to various magnetic fields, obtained when the reproducing light beam is radiated and the temperature is raised by actually using the recording and reproducing apparatus for the magneto-optical recording medium. Therefore, the hysteresis curves, Hs, Hn, and Hr to be applied are observed by using a practical magneto-optical recording and reproducing apparatus while radiating the reproducing light beam having the power for reproduction.
According to the MAMMSO, owing to the provision of the gate layer as described above, only one magnetic domain is allowed to emerge on the gate layer 16, or it can be transferred to the gate layer 16 even when a plurality of magnetic domains exist in the information-recording layer. Further, the one minute magnetic domain having been transferred to the gate layer 16 can be transferred to the magnetic domain-magnifying and reproducing layer 3, and it can be magnified and detected (reproduced) by using the reproducing magnetic field. Therefore, the minute magnetic domain formed in accordance with the optical magnetic field modulation system can be subjected to reproduction at a high resolving power and at high S/N.
The principle of the MAMMOS has been explained by illustrating the gate layer as the magnetic layer which undergoes temperature distribution of the gate layer generated in the reproducing light beam spot, in which the magnetic domain in the information-recording layer is transferred to the gate layer in a high temperature area having a temperature higher than a predetermined temperature. However, it is possible to use a magnetic layer which undergoes the temperature distribution in the gate layer generated in the reproducing light beam spot, in which the magnetic domain in the information-recording layer is transferred to the gate layer in a low temperature area having a temperature lower than a predetermined temperature. Alternatively, it is possible to use a magnetic layer which undergoes the temperature distribution in the gate layer generated in the reproducing light beam spot, in which the magnetic domain in the information-recording layer is transferred to the gate layer in a predetermined temperature range. In the foregoing magneto-optical recording medium, the recording layer and the reproducing layer may be magnetostatically coupled to one another with a non-magnetic layer interposed between the recording layer and the reproducing layer so that transfer may be effected from the recording layer to the reproducing layer.